A hip joint comprises a socket or acetabulum and a femoral head or ball received in the acetabulum. As such, the hip joint is a ball and socket joint which provides universal motion.
Various diseases, such as osteoarthritis, are known to attack the hip joint and, when this occurs, it may be necessary to utilize an appropriate hip joint prosthesis to replace the femoral head and the acetabulum. Replacement may also be necessary in other circumstances, such as in the instance of certain hip joint fractures.
Deterioration of the acetabulum requires that an acetabular cup be mounted in the acetbular cavity to provide a socket for receiving the prosthetic femoral head. If the medial wall of the acetabulum is sufficiently weakened, it has previously been found desirable to protect this wall with a protrusio shell or ring. A typical construction of a protrusio ring is disclosed in U.S. Pat. No. 4,623,352. In that instance, before implanting, a plastic acetabular cup is first interlocked with the protrusio ring, then the two components in unitary fashion are cemented into place on the acetabulum.
Another example of a protective implant for the acetabulum is disclosed in U.S. Pat. No. 3,918,102. In this instance, a generally annular shaped support member bears firmly against bone tissue surrounding the acetabular cavity. A spherical portion of the support member is configured for reception with the bone in the acetabular cavity but defines a substantially sized opening in its central regions. Bone cement is used to affix the support member and its spherical portion to the acetabulum. Subsequently, a hemispherical socket is inserted into the support member and is also affixed to the acetabulum through the central opening in the support member.
The use of acrylic cement for fixation of implants to bone was made popular by orthopedic pioneer Sir John Charnley and continues to be widely used. Although cement was fully accepted in the early years of total hip arthroplasty, its use is being subjected more and more to question. As early as 1970, loosening of hip joint components was recognized as a worrisome problem and, by the latter part of that decade, loosening had been identified as the most common complication in total hip arthroplasty. As related by Park et al in their paper entitled "Pre-Coated Orthopedic Implants with Bone Cement" published in the Journal of Biomaterials, Medical Devices and Artificial Organs, Vol. 7, No. 1 (1979) on page 42:
"Some problems can be attributed to the following intrinsic properties of the bone cement itself: PA1 1. The monomer and additives are toxic. PA1 2. The polymerization is exothermic and accompanied by a volumetric shrinkage. PA1 3. The strength and stiffness are much lower than bone and implant. PA1 4. Short working time and no recourse to correct mistakes (extremely hard to remove after cured, particularly from a long intramedullary site). PA1 5. Abrasion particulate of cement may cause bone necrosis causing the loosening of implant."
Additional problems with cement have been reported by Miller and Johnson in their paper entitled: "Advances in Cementing Techniques In Total Hip Arthroplasty", being Chapter 19 in The Art of Total Hip Arthroplasty edited by William Thomas Stillwell, M.D., and published in 1987 by Grune and Stratton, Inc. In that paper, Miller and Johnson state that the exothermic character of polymerization of the acrylic cement suggests the possibility of bone injury and note on page 277 that "even Charnley postulated that thermal damage to the endosteal bone of the femur is to be expected." They further indicate that carcinogenesis has also been suggested although no clear evidence has emerged from clinical studies to support such a suggestion. Thus, it is clear that the use of acrylic cement for fixation of orthopedic implants is becoming more and more suspect, thereby inviting alternative constructions and procedures.